1. Field of the Invention
The present invention relates to an apparatus producing chromatic dispersion, and which can be used to compensate for chromatic dispersion accumulated in an optical fiber transmission line. More specifically, the present invention relate to an apparatus which uses a virtually imaged phased array to produce chromatic dispersion.
2. Description of the Related Art
FIG. 1(A) is a diagram illustrating a conventional fiber optic communication system, for transmitting information via light. Referring now to FIG. 1(A), a transmitter 30 transmits pulses 32 through an optical fiber 34 to a receiver 36. Unfortunately, chromatic dispersion, also referred to as xe2x80x9cwavelength dispersionxe2x80x9d, of optical fiber 34 degrades the signal quality of the system.
More specifically, as a result of chromatic dispersion, the propagating speed of a signal in an optical fiber depends on the wavelength of the signal. For example, when a pulse with a longer wavelength (for example, a pulse with wavelengths representing a xe2x80x9credxe2x80x9d color pulse) travels faster than a pulse with a shorter wavelength (for example, a pulse with wavelengths representing a xe2x80x9cbluexe2x80x9d color pulse), the dispersion is typically referred to as xe2x80x9cnormalxe2x80x9d dispersion. By contrast, when a pulse with a shorter wavelength (such as a blue color pulse) is faster than a pulse with a longer wavelength (such as a red color pulse), the dispersion is typically referred to as xe2x80x9canomalousxe2x80x9d dispersion.
Therefore, if pulse 32 consists of red and blue color pulses when emitted from transmitter 30, pulse 32 will be split as it travels through optical fiber 34 so that a separate red color pulse 38 and a blue color pulse 40 are received by receiver 36 at different times. FIG. 1(A) illustrates a case of xe2x80x9cnormalxe2x80x9d dispersion, where a red color pulse travels faster than a blue color pulse.
As another example of pulse transmission, FIG. 1(B) is a diagram illustrating a pulse 42 having wavelength components continuously from blue to red, and transmitted by transmitter 30. FIG. 1(C) is a diagram illustrating pulse 42 when arrived at receiver 36. Since the red component and the blue component travel at different speeds, pulse 42 is broadened in optical fiber 34 and, as illustrated by FIG. 1(C), is distorted by chromatic dispersion. Such chromatic dispersion is very common in fiber optic communication systems, since all pulses include a finite range of wavelengths.
Therefore, for a fiber optic communication system to provide a high transmission capacity, the fiber optic communication system must compensate for chromatic dispersion.
FIG. 2 is a diagram illustrating a fiber optic communication system having an opposite dispersion component to compensate for chromatic dispersion. Referring now to FIG. 2, generally, an opposite dispersion component 44 adds an xe2x80x9coppositexe2x80x9d dispersion to a pulse to cancel dispersion caused by traveling through optical fiber 34.
There are conventional devices which can be used as opposite dispersion component 44. For example, FIG. 3 is a diagram illustrating a fiber optic communication system having a dispersion compensation fiber which has a special cross-section index profile and thereby acts as an opposite dispersion component, to compensate for chromatic dispersion. Referring now to FIG. 3, a dispersion compensation fiber 46 provides an opposite dispersion to cancel dispersion caused by optical fiber 34. However, a dispersion compensation fiber is expensive to manufacture, and must be relatively long to sufficiently compensate for chromatic dispersion. For example, if optical fiber 34 is 100 km in length, then dispersion compensation fiber 46 should be approximately 20 to 30 km in length.
FIG. 4 is a diagram illustrating a chirped grating for use as an opposite dispersion component, to compensate for chromatic dispersion. Referring now to FIG. 4, light traveling through an optical fiber and experiencing chromatic dispersion is provided to an input port 48 of an optical circulator 50 provides the light to chirped grating 52. Chirped grating 52 reflects the light back towards circulator 50, with different wavelength components reflected at different distances along chirped grating 52 so that different wavelength components travel different distances to thereby compensate for chromatic dispersion. For example, chirped grating 52 can be designed so that longer wavelength components are reflected at a farther distance along chirped grating 52, and thereby travel a farther distance than shorter wavelength components. Circulator 50 then provides the light reflected from chirped grating 52 to an output port 54. Therefore, chirped grating 52 can add opposite dispersion to a pulse.
Unfortunately, a chirped grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system. Instead, a chirped grating with a circulator, as in FIG. 4, is more suitable for use when a single channel is transmitted through a fiber optic communication system.
FIG. 5 is a diagram illustrating a conventional diffraction grating, which can be used in producing chromatic dispersion. Referring now to FIG. 5, a diffraction grating 56 has a grating surface 58. Parallel lights 60 having different wavelengths are incident on grating surface 58. Lights are reflected at each step of grating surface 58 and interfere with each other. As a result, lights 62, 64 and 66 having different wavelengths are output from diffraction grating 56 at different angles. A diffraction grating can be used in a spatial grating pair arrangement, as discussed in more detail below, to compensate for chromatic dispersion.
More specifically, FIG. 6(A) is a diagram illustrating a spatial grating pair arrangement for use as an opposite dispersion component, to compensate for chromatic dispersion. Referring now to FIG. 6(A), light 67 is diffracted from a first diffraction grating 68 into a light 69 for shorter wavelength and a light 70 for longer wavelength. These lights 69 and 70 are then diffracted by a second diffraction grating 71 into lights propagating in the same direction. As can be seen from FIG. 6(A), wavelength components having different wavelengths travel different distances, to add opposite dispersion and thereby compensate for chromatic dispersion. Since longer wavelengths (such as lights 70) travel longer distance than shorter wavelengths (such as lights 69), a spatial grating pair arrangement as illustrated in FIG. 6(A) has anomalous dispersion.
FIG. 6(B) is a diagram illustrating an additional spatial grating pair arrangement for use as an opposite dispersion component, to compensate for chromatic dispersion. As illustrated in FIG. 6(B), lenses 72 and 74 are positioned between first and second diffraction gratings 68 and 71 so that they share one of the focal points. Since longer wavelengths (such as lights 70) travel shorter distance than shorter wavelengths (such as lights 69), a spatial grating pair arrangement as illustrated in FIG. 6(B) has normal dispersion.
A spatial grating pair arrangement as illustrated in FIGS. 6(A) and 6(B) is typically used to control dispersion in a laser resonator. However, a practical spatial grating pair arrangement cannot provide a large enough dispersion to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. More specifically, the angular dispersion produced by a diffraction grating is usually extremely small, and is typically approximately 0.05 degrees/nm. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, first and second gratings 68 and 71 would have to be separated by very large distances, thereby making such a spatial grating pair arrangement impractical.
Therefore, it is an object of the present invention to provide an apparatus which produces chromatic dispersion, and which is practical for compensating for chromatic dispersion accumulated in an optical fiber.
Objects of the present invention are achieved by providing an apparatus which includes a device herein referred to as a xe2x80x9cvirtually imaged phased arrayxe2x80x9d, xe2x80x9cVIPAxe2x80x9d or xe2x80x9cVIPA generatorxe2x80x9d. The VIPA generator produces a light propagating away from the VIPA generator. The apparatus also includes a mirror or reflecting surface which returns the light back to the VIPA generator to undergo multiple reflection inside the VIPA generator.
Objects of the present invention are achieved by providing an apparatus comprising a VIPA generator and a reflecting surface. The VIPA generator receives an input light at a respective wavelength and produces a corresponding collimated output light traveling from the VIPA generator in a direction determined by the wavelength of the input light. The reflecting surface reflects the output light back to the VIPA generator. The reflecting surface has different curvatures at different positions along a direction perpendicular to an angular dispersion direction of the VIPA generator, or a plane which includes the traveling directions of collimated output light from the VIPA generator for input light at different wavelengths.
Objects of the present invention are also achieved by providing an apparatus which includes a VIPA generator, a reflecting surface, and a lens. The VIPA generator receives an input light at a respective wavelength and produces a corresponding collimated output light traveling from the VIPA generator in a direction determined by the wavelength of the input light, the output light thereby being spatially distinguishable from an output light produced for an input light at a different wavelength. The reflecting surface has a cone shape, or a modified cone shape. The lens focuses the output light traveling from the VIPA generator onto the reflecting surface so that the reflecting surface reflects the output light, the reflected light being directed by the lens back to the VIPA generator. The modified cone shape can be designed so that the apparatus provides a uniform chromatic dispersion to light in the same channel of a wavelength division multiplexed light.
Objects of the present invention are achieved by providing an apparatus comprising an angular dispersive component and a reflecting surface. The angular dispersive component has a passage area to receive light into, and to output light from, the angular dispersive component. The angular dispersive component receives, through the passage area, an input light having a respective wavelength within a continuous range of wavelengths, and causes multiple reflection of the input light to produce self-interference that forms a collimated output light which travels from the angular dispersive component along a direction determined by the wavelength of the input light and is thereby spatially distinguishable from an output light formed for an input light having any other wavelength within the continuous range of wavelengths. The reflecting surface reflects the output light back to the angular dispersive component to undergo multiple reflection in the angular dispersive component and then be output from the passage area. The reflecting surface has different curvatures at different positions along a direction which is perpendicular to a plane which includes the travel direction of collimated output light from the angular dispersive component for input light at different wavelengths.
Moreover, objects of the present invention are achieved by providing an apparatus which includes an angular dispersive component and a reflecting surface. The angular dispersive component has a passage area to receive light into, and to output light from, the angular dispersive component. The angular dispersive component receives, through the passage area, a line focused input light and causes multiple reflection of the input light to produce self-interference that forms a collimated output light which travels from the angular dispersive component along a direction determined by the wavelength of the input light and is thereby spatially distinguishable from an output light formed for an input light having a different wavelength. The reflecting surface reflects the output light back to the angular dispersive component to undergo multiple reflection in the angular dispersive component and then be output from the passage area. The reflecting surface has different curvatures at different positions along a direction which is perpendicular to a plane which includes the travel direction of collimated output light from the angular dispersive component for input light at different wavelengths.
Objects of the present invention are still further achieved by providing an apparatus comprising first and second reflecting surfaces, and a mirror. The second reflecting surface has a reflectivity which causes a portion of light incident thereon to be transmitted therethrough. An input light at a respective wavelength is focused into a line. The first and second reflecting surfaces are positioned so that the input light radiates from the line to be reflected a plurality of times between the first and second reflecting surfaces and thereby cause a plurality of lights to be transmitted through the second reflecting surface. The plurality of transmitted lights interfere with each other to produce a collimated output light which travels from the second reflecting surface along a direction determined by the wavelength of the input light, and is thereby specially distinguishable from an output light formed for an input light having a different wavelength. The mirror surface reflects output the light back to the second reflecting surface to pass through the second reflecting surface and undergo multiple reflection between the first and second reflecting surfaces. The mirror surface has different curvatures at different positions along a direction which is perpendicular to a plane which includes the travel direction of collimated output light from the second reflecting surface for input light at different wavelengths.
Objects of the present invention are also achieved by providing an apparatus which includes a VIPA generator, a lens, first and second mirrors, and a wavelength filter. The VIPA generator receives a line focused wavelength division multiplexed (WDM) light including light at first and second wavelengths, and produces collimated first and second output lights corresponding, respectively, to the first and second wavelengths. The first and second output lights travel from the VIPA generator in first and second directions, respectively, determined by the first and second wavelengths, respectively. The lens focuses the first and second output lights traveling from the VIPA generator. The first and second mirrors each having a cone shape or a modified cone shape for producing a uniform chromatic dispersion. The wavelength filter filters light focused by the lens so that light at the first wavelength is focused to the first mirror and reflected by the first mirror, and light at the second wavelength is focused to the second mirror and reflected by the second mirror. The reflected first and second lights are directed by the wavelength filter and the lens back to the VIPA generator.
Moreover, objects of the present invention are achieved by causing the input light to have a double-hump shaped far field distribution. For example, a phase mask can be provided on an input fiber, or on a surface of the VIPA generator, to cause the input light to have a double-hump shaped far field distribution.